Method for fabricating an environmental barrier coating on a ceramic matrix composite

ABSTRACT

A method of fabricating a coating includes providing a ceramic matrix composite that includes SiC fibers disposed in a SiC matrix, depositing a base slurry on the ceramic matrix composite, wherein the base slurry contains powders of a metal oxide, at least one of silicon carbide, silicon nitride, or free silicon, and barium-magnesium-aluminosilicate in a first carrier fluid, drying the deposited base slurry to produce a base green layer, depositing a transition slurry on the base green layer, wherein the transition slurry contains powders of a metal oxide, at least one of silicon carbide, silicon nitride, or free silicon, at least one of zirconium carbide, zirconium nitride, or zirconium oxide, and barium-magnesium-aluminosilicate in a second carrier fluid, drying the deposited transition slurry to produce a transition green layer, and forming a consolidated coating on the ceramic matrix composite by heating the base green layer and the at least one transition green layer to cause chemical reactions that convert the powders to metal-silicon-oxygen rich phase and metal-zirconium-oxygen rich phase.

BACKGROUND

Components in a gas turbine engine often include barrier coatings toprotect the underlying component from the effects of the severeoperating environment. Barrier coatings are available in numerousvarieties, which can include thermal barrier coatings and environmentalbarrier coatings. Thermal barrier coatings are typically designed formaximizing thermal insulation of a component from the surroundinghigh-temperature environment. Environmental barrier coatings aretypically designed for maximizing resistance of infiltration or attackby environmental substances.

SUMMARY

A method of fabricating a coating according to an example of the presentdisclosure includes providing a ceramic matrix composite that includesSiC fibers disposed in a SiC matrix. A base slurry is deposited on theceramic matrix composite and is dried to produce a base green layer. Atransition slurry is deposited on the base green layer and is dried toproduce a transition green layer. The base slurry contains powders of ametal oxide, at least one of silicon carbide, silicon nitride, or freesilicon, barium-magnesium-aluminosilicate, and a first carrier fluid.The transition slurry contains powders of a metal oxide, at least one ofsilicon carbide, silicon nitride, or free silicon, at least one ofzirconium carbide, zirconium nitride, or zirconium oxide,barium-magnesium-aluminosilicate, and a second carrier fluid. Aconsolidated coating is formed on the ceramic matrix composite byheating the base green layer and the at least one transition green layerto cause chemical reactions that convert the powders of the at least oneof the silicon carbide, silicon nitride, or free silicon, and the atleast one of the zirconium carbide, zirconium nitride, or zirconiumoxide to, respectively, metal-silicon-oxygen rich phase andmetal-zirconium-oxygen rich phase.

In a further embodiment of any of the foregoing embodiments, the metaloxide of the base slurry and the transition slurry is selected from thegroup consisting of HfO2, Y2O3, Yb2O3, Lu2O3, oxides of La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the metaloxide of at least one of the base slurry or the transition slurry isHfO₂.

In a further embodiment of any of the foregoing embodiments, the metaloxide of the base slurry and the transition slurry is HfO₂.

A further embodiment of any of the foregoing embodiments includesapplying a topcoat on the consolidated coating, the topcoat beingselected from the group consisting of rare earth silicates, HfO₂, andcombinations thereof.

A further embodiment of any of the foregoing embodiments includesapplying a topcoat on the consolidated coating, wherein the topcoat isHfO₂.

A further embodiment of any of the foregoing embodiments includesapplying a topcoat on the consolidated coating, wherein the topcoat isGd₂Zr₂O₇.

In a further embodiment of any of the foregoing embodiments, themetal-silicon-oxygen rich phase is HfSiO₄ and the metal-zirconium-oxygenrich phase is HfZrO₄ or HfO₂ and ZrO₂.

In a further embodiment of any of the foregoing embodiments, theconsolidated coating has a porosity, by volume, of 1% to 20%.

In a further embodiment of any of the foregoing embodiments, the firstcarrier fluid and the second carrier fluid are water.

In a further embodiment of any of the foregoing embodiments, the heatingis conducted at 1482° C.+/−125° C. in air for at least 1 hour.

A method of fabricating a coating according to an example of the presentdisclosure includes providing a ceramic matrix composite that includesSiC fibers disposed in a SiC matrix. A base slurry is deposited on theceramic matrix composite and is dried to produce a base green layer. Atransition slurry is deposited on the base green layer and is dried toproduce a transition green layer. The base slurry contains, in parts byweight, 35-60 of a metal oxide powder, 5-20 of at least one of siliconcarbide powder, silicon nitride powder, or free silicon powder, and0.2-10 of barium-magnesium-aluminosilicate powder in a first carrierfluid. The transition slurry contains, in parts by weight, 35-60 of ametal oxide powder, an amount X₁ of at least one of silicon carbidepowder, silicon nitride powder, or free silicon powder, an amount X₂ ofat least one of zirconium carbide powder, zirconium nitride powder, orzirconium oxide powder, and 0.2-10 of barium-magnesium-aluminosilicatepowder in a second carrier fluid. The total amount of X₁+X₂ is 5-20, andthe amount X₁ is decreased and the amount of X₂ is increased through thedeposition of the transition slurry. A consolidated coating if formed onthe ceramic matrix composite by heating the base green layer and thetransition green layer.

In a further embodiment of any of the foregoing embodiments, through thedeposition of the transition slurry the amount X₁ is decreased to, andthen held at, a non-zero amount, followed by decreasing the non-zeroamount to zero.

In a further embodiment of any of the foregoing embodiments, the amountX₁ is decreased and the amount of X₂ is increased cooperatively suchthat through the deposition of the transition slurry X₁+X₂ is constant.

In a further embodiment of any of the foregoing embodiments, the amountX₁ is linearly decreased and the amount of X₂ is linearly increased.

In a further embodiment of any of the foregoing embodiments, the metaloxide of the base slurry and the transition slurry is selected from thegroup consisting of HfO₂, Y₂O₃, Yb₂O₃, Lu₂O₃, oxides of La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and combinations thereof.

A further embodiment of any of the foregoing embodiments includesapplying a topcoat on the consolidated coating, the topcoat beingselected from the group consisting of HfO₂, Gd₂Zr₂O₇, and combinationsthereof.

A gas turbine engine article according to an example of the presentdisclosure includes a ceramic matrix composite substrate that includesSiC fibers disposed in a SiC matrix, and a coating disposed on, and incontact with, the ceramic matrix composite. The coating includes, byvolume percent, 5% to 20% of barium-magnesium-aluminosilicate, and aremainder of a metal-silicon-oxygen rich phase and ametal-zirconium-oxygen rich phase dispersed through thebarium-magnesium-aluminosilicate.

In a further embodiment of any of the foregoing embodiments, themetal-silicon-oxygen rich phase is HfSiO₄ and the metal-zirconium-oxygenrich phase is HfZrO₄ or HfO₂ and ZrO₂.

In a further embodiment of any of the foregoing embodiments, the coatinghas a porosity, by volume, of 1% to 20%.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 depicts a method for fabricating an environmental barriercoating.

FIG. 2 illustrates a sectioned view of an environmental barrier coating.

FIG. 3 depicts another step that can be used in the method, to apply atopcoat.

DETAILED DESCRIPTION

Environmental barrier coatings (“EBC”) can be used in components of gasturbine engines, such as airfoils, combustors, and outer air seals, toprotect underlying substrates from infiltration and/or attack byenvironmental substances. One such substance iscalcium-magnesium-aluminosilicate, which is known as “CMAS.” Debris suchas dirt, sand, and other foreign substances entrained in gas flowthrough an engine can include or produce CMAS. The CMAS is viscous,possibly molten, and can wick into surfaces and ultimately causespallation. It has been discovered that the design of an EBC is furthercomplicated by a competing factor of thermal compatibility with theunderlying substrate and, in particular, ceramic matrix composite(“CMC”) substrates composed of SiC/SiC (SiC fibers in SiC matrix). Thecoefficient of thermal expansion of SiC/SiC CMC substantially differsfrom known EBCs. As a result, thermal cycling can cause strain betweenan EBC and its substrate, which could have potential to reducedurability of the EBC. In this regard, as will be described furtherherein, the disclosed EBC is configured to more closely match thecoefficient of thermal expansion of SiC/SiC CMCs while maintainingfunctionality as an environmental barrier.

FIG. 1 schematically depicts an example method 20 of fabricating an EBC.In general, the method will be described with reference to steps 22, 24,26, 28, 30, and 32. It is to be understood, however, that the steps arenot necessarily separate or distinct and that steps may overlap in timeor space.

The method 20 begins at step 22 with the provision of a ceramic matrixcomposite 40 (“CMC 40”). The CMC 40 includes SiC fibers 40 a disposed ina SiC matrix 40 b. The SiC fibers 40 a may be provided in a fiberstructure, such as but not limited to, woven structures, non-wovenstructures, unidirectional structures, or combinations of differentstructures that are stacked in layers. The provision of the CMC 40 mayinclude furnishing the CMC 40 as a pre-fabricated substrate or,alternatively, fabricating the CMC 40 in whole or in part, such asthrough a ceramic infiltration process and/or pyrolysis process.

At step 24 a base slurry 42 is deposited on the CMC 40. The base slurry42 contains powders of a metal oxide, at least one of silicon carbide,silicon nitride, or free silicon, and barium-magnesium-aluminosilicatein a first carrier fluid. As used herein, free silicon refers to siliconthat is not bonded to other, different elements. For example, the metaloxide is selected from HfO₂, Y₂O₃, Yb₂O₃, Lu₂O₃, oxides of La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and combinations thereof. In onefurther example, the first carrier fluid is water. Alternatively, othercarrier fluids, such as polar or non-polar solvents can be used. Themethod of deposition is not particularly limited and can be, but is notlimited to, air brushing, dipping, brushing, ultrasonic spraying, orsuspension plasma spraying.

In one particular example, the base slurry 42 contains, in parts byweight, of the metal oxide powder, 5-20 of the silicon carbide powder,silicon nitride powder, and/or free silicon powder, and 0.2-10 of thebarium-magnesium-aluminosilicate powder in the first carrier fluid. Theweight of the barium-magnesium-aluminosilicate may include bound waterweight. Unless stated otherwise, all composition amounts here are givenin parts by weight. The metal oxide powder has an average powderparticle size of approximately 1 micrometer to 2 micrometers, thesilicon carbide, silicon nitride, and/or free silicon has an averagepowder particle size of approximately 1 micrometer to 2 micrometers, andthe barium magnesium-aluminosilicate powder has powder particles size of−325 mesh, i.e., less than 45 micrometers. The amount of the firstcarrier fluid can be varied to adjust viscosity. For example, the baseslurry 42 contains about 10 to 59 parts by weight of the first carrierfluid.

At step 26 the deposited base slurry 42 is dried to produce a base greenlayer 44. For instance, the drying can be conducted by heating the CMC40 to evaporate the first carrier fluid. Additionally or alternatively,the drying can be conducted by permitting the deposited base slurry 42to dwell at ambient temperature conditions, typically about 65° C. tountil the first carrier fluid fully or substantially fully evaporates.In other examples, the rate of evaporation may be rapid such that littleor no dwell is needed and the first carrier fluid fully or substantiallyfully evaporates as the base slurry 42 is deposited.

At step 28 a transition slurry 46 is deposited on the base green layer44. The transition slurry 46 contains powders of a metal oxide, at leastone of silicon carbide, silicon nitride, or free silicon, at least oneof zirconium carbide, zirconium nitride, or zirconium oxide, andbarium-magnesium-aluminosilicate in a second carrier fluid. For example,the metal oxide is also selected from HfO₂, Y₂O₃, Yb₂O₃, Lu₂O₃, oxidesof La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and combinationsthereof. In one example, the metal oxide of the transition slurry 46 isthe same as the metal oxide of the base slurry 42. In one furtherexample, the second carrier fluid is also water. Alternatively, othercarrier fluids, such as polar or non-polar solvents can be used. Again,the method of deposition is not particularly limited and can be, but isnot limited to, air brushing, dipping, brushing, ultrasonic spraying, orsuspension plasma spraying.

In one particular example, the transition slurry 46 contains, in partsby weight, 35-60 of the metal oxide powder, 5-20 of a combined amount ofsilicon carbide, silicon nitride, and/or free silicon powder andzirconium carbide powder, zirconium nitride powder, and/or zirconiumoxide powder, and 0.2-10 of the barium-magnesium-aluminosilicate powderin the second carrier fluid. The metal oxide powder has an averagepowder particle size of approximately 1 micrometer to 2 micrometers, thesilicon carbide, silicon nitride, and/or free silicon has an averagepowder particle size of approximately 1 micrometer to 2 micrometers, thezirconium carbide, zirconium nitride, or zirconium oxide has an averagepowder particle size of approximately 5 micrometers to 6 micrometers,and the barium magnesium-aluminosilicate powder has powder particlessize of −325 mesh, i.e., less than 45 micrometers. The amount of thesecond carrier fluid can be varied to adjust viscosity. For example, thetransition slurry 46 contains about 10 to 59 parts by weight of thesecond carrier fluid.

At step 30, similar to step 26, the deposited transition slurry 46 isdried to produce a transition green layer 48. For instance, the dryingcan be conducted by heating the CMC 40 to evaporate the second carrierfluid. Additionally or alternatively, the drying can be conducted bypermitting the deposited transition slurry to dwell at ambienttemperature conditions, typically about 65° C. to 75° C., until thesecond carrier fluid fully or substantially fully evaporates. In otherexamples, the rate of evaporation may be rapid such that little or nodwell is needed and the second carrier fluid fully or substantiallyfully evaporates as the transition slurry 46 is deposited. In oneexample, the final transition green layer 48 is provided at a thicknessof approximately 100 micrometers to approximately 525 micrometers.

Upon consolidation, described further below, the base slurry 42 willprovide a composition that closely matches the coefficient of thermalexpansion of the CMC 40, while the transition slurry 46 will provide acomposition with higher coefficient of thermal expansion and potentialfor enhanced CMAS resistance. In this regard, carbide of the base slurry42 is silicon carbide while the transition slurry 46 may containzirconium carbide, zirconium nitride, and/or zirconium oxide. Forinstance, in one example, the transition slurry 46 initially containshigh level of silicon carbide and low level of zirconium carbide,zirconium nitride, or zirconium oxide. As the thickness of thetransition green layer 48 increases, the composition of the transitionslurry 46 is changed to reduce the level of silicon carbide and increasethe level of zirconium carbide, zirconium nitride, or zirconium oxide.In one example, the final transition slurry 46 applied has little or nosilicon carbide.

In one example, the transition slurry 46 contains an amount X₁ ofsilicon carbide powder, an amount X₂ of zirconium carbide, zirconiumnitride, or zirconium oxide powder, and 0.2-10 ofbarium-magnesium-aluminosilicate powder in the second carrier fluid, andthe total amount of X₁+X₂ is 5-20. The amount X₁ is decreased and theamount of X₂ is increased through the deposition of the transitionslurry 46.

The amounts X₁ and X₂ can be adjusted through the deposition by mixingseveral slurries together. For instance, at the conclusion of thedeposition of the base slurry 42, when the base green layer 44 has beendeposited to a desired thickness, a second slurry can be mixed into thebase slurry 42 to produce the transition slurry 46. The second slurrycan contain of the metal oxide powder, 5-20 of zirconium carbide,zirconium nitride, or zirconium oxide powder, and 0.2-10 of thebarium-magnesium-aluminosilicate powder in the second carrier fluid. Asthe deposition proceeds, additional amount of the second slurry can bemixed into the base slurry 42 to “dilute” the base slurry 42 and therebyin essence decrease the amount of silicon carbide powder being depositedand increase the amount of zirconium carbide, zirconium nitride, orzirconium oxide powder being deposited. Alternatively, rather thandilute the base slurry 42, several transition slurries 46 can beprepared with controlled, different amounts of X₁ and X₂. The transitionslurries 46 can then be deposited successively, starting from the onewith the highest amount X₁ and the lowest amount of X₂, followed by theslurry with the next highest amount X₁ and the next lowest amount of X₂,and so on and so forth. In another alternative, the composition of thetransition slurry 46 is computer-controlled and can include feedingcontrolled amounts of slurry and/or powders to adjust composition of thetransition slurry 46.

Using the above techniques, the amount X₁ can be decreased and theamount of X₂ increased according to a predefined profile. For example,through the deposition of the transition slurry 46 the amount X₁ isdecreased to, and then held at, a non-zero amount, followed bydecreasing the non-zero amount to zero. This provides a step-wise changein the composition, and several step changes can be used. In a furtherexample, the amount X₁ is decreased and the amount of X₂ is increasedcooperatively such that through the deposition of the transition slurry46, X₁+X₂ is constant. This provides the same amount of carbide throughthe thickness of the transition green layer 48. In one additionalexample the amount X₁ is linearly decreased and the amount of X₂ islinearly increased. This provides a relatively, smooth, linear change incomposition.

At step 32, a consolidated coating 50 is formed on the CMC 40 by heatingthe base green layer 44 and the transition green layer 48 to causechemical reactions that convert the powders of the silicon carbide,silicon nitride, and/or free silicon and the zirconium carbide,zirconium nitride, or zirconium oxide to, respectively,metal-silicon-oxygen rich phase and metal-zirconium-oxygen rich phase.As an example, the heating is conducted at 1482° C.+/−125° C. in air forat least 1 hour and up to about 24 hours. For instance, the siliconcarbide, silicon nitride, and/or free silicon and the zirconium carbide,zirconium nitride, or zirconium oxide first oxidize to form SiO₂ andZrO₂, respectively. These oxides then react with the metal oxide to formthe metal-silicon-oxygen rich phase and the metal-zirconium-oxygen richphase. The reaction of the oxides may also involve a volume expansion,which may facilitate increasing density of the coating 50. Moreover,forming the metal-silicon-oxygen rich phase and themetal-zirconium-oxygen rich phase in situ rather than by using powdersof these phases avoids spraying or handling the phases. To the extentthat any silicon carbide and/or zirconium carbide, zirconium nitride, orzirconium oxide remains after the heating, the silicon carbide and/orzirconium carbide, zirconium nitride, or zirconium oxide may serve as anoxygen getter in the coating 50 by reacting with oxygen gas thatinfiltrates.

For instance, the coating 50 has a composition, by volume, of 1% to 20%of barium-magnesium-aluminosilicate and a remainder of ametal-silicon-oxygen rich phase and a metal-zirconium-oxygen rich phasedispersed through the barium-magnesium-aluminosilicate. The metal of themetal-silicon-oxygen rich phase and the metal-zirconium-oxygen richphase is the metal of the metal oxide in the slurry powders, i.e.,hafnium from HfO₂, yttrium from Y₂O₃, ytterbium from Yb₂O₃, lutetiumfrom Lu₂O₃, or the metals La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm from the corresponding oxides. For example, if HfO₂ is used, themetal-silicon-oxygen rich phase is HfSiO₄ and the metal-zirconium-oxygenrich phase is HfZrO₄. The coating 50 may also have residual porosity.For instance, the coating has a porosity, by volume, of 1% to 20%.

The method 22 may be used for an initial or original manufacture of thecoating 50, as a repair to patch or replace an existing EBC, or even toapply the coating 50 as a bond layer between other coatings orcomponents.

FIG. 2 shows a sectioned view of the coating 50. The coating 50 includessub-layers 50 a, 50 b, and 50 c, denoted by dashed lines. The sub-layer50 a is in contact with the CMC 40 and is the layer that results fromthe base slurry 42, while sub-layers 50 b and 50 c overlay the sub-layer50 a and are the layers that result from the transition slurry 46. Inthese regards, the sub-layer 50 a has a barium-magnesium-aluminosilicatematrix 60 a and the metal-silicon-oxygen rich phase 60 b dispersed therethrough; the sub-layer 50 b has the barium-magnesium-aluminosilicatematrix 60 a and the metal-silicon-oxygen rich phase 60 b andmetal-zirconium-oxygen rich phase 60 c dispersed there through; and thesub-layer 50 c has the barium-magnesium-aluminosilicate matrix 60 a andthe metal-zirconium-oxygen rich phase 60 c dispersed there through. Thatis, the sub-layer 50 a directly in contact with the CMC 40 hasmetal-silicon-oxygen rich phase 60 b, which has a coefficient of thermalexpansion that closely matches the CMC 40. As an example, the CMC 40 hasa coefficient of thermal expansion of approximately 4×10⁻⁶/° C., whilethe metal-silicon-oxygen rich phase 60 b has a coefficient of thermalexpansion of approximately 4.5×10⁻⁶/° C. The sub-layer 50 b provides atransition or graded layer in which the metal-silicon-oxygen rich phase60 b decreases and the metal-zirconium-oxygen rich phase 60 c increases.The sub-layer 50 c has metal-zirconium-oxygen rich phase 60 bc, whichhas a higher coefficient of thermal expansion that the CMC 40 butenhanced potential for CMAS resistance.

FIG. 3 depicts an additional step 34 that can be used in the method 22,after step 32. The step 34 includes applying a topcoat 62 on theconsolidated coating 50, for additional CMAS resistance. For example,the topcoat 62 is selected from rare earth silicates, HfO₂, orcombinations thereof. In one additional example, the topcoat 62 isGd₂Zr₂O₇. The topcoat 62 can be applied by air plasma spray or electronbeam physical vapor deposition, but other methods may alternatively beused.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthis disclosure. The scope of legal protection given to this disclosurecan only be determined by studying the following claims.

What is claimed is:
 1. A method of fabricating a coating, the methodcomprising: providing a ceramic matrix composite that includes SiCfibers disposed in a SiC matrix; depositing a base slurry on the ceramicmatrix composite, wherein the base slurry contains, in parts by weight,35-60 of a metal oxide powder, 5-20 of at least one of silicon carbidepowder, silicon nitride powder, or free silicon powder, and 0.2-10 ofbarium-magnesium-aluminosilicate powder in a first carrier fluid; dryingthe deposited base slurry to produce a base green layer; depositing atransition slurry on the base green layer, wherein the transition slurrycontains, in parts by weight, 35-60 of a metal oxide powder, an amountX₁ of at least one of silicon carbide powder, silicon nitride powder, orfree silicon powder, an amount X₂ of at least one of zirconium carbidepowder, zirconium nitride powder, or zirconium oxide powder, and 0.2-10of barium-magnesium-aluminosilicate powder in a second carrier fluid,the total amount of X₁+X₂ is 5-20, and the amount X₁ is decreased andthe amount of X₂ is increased through the deposition of the transitionslurry; drying the deposited transition slurry to produce a transitiongreen layer; and forming a consolidated coating on the ceramic matrixcomposite by heating the base green layer and the transition greenlayer.
 2. The method as recited in claim 1, wherein through thedeposition of the transition slurry the amount X₁ is decreased to, andthen held at, a non-zero amount, followed by decreasing the non-zeroamount to zero.
 3. The method as recited in claim 1, wherein the amountX₁ is decreased and the amount of X₂ is increased cooperatively suchthat through the deposition of the transition slurry X₁+X₂ is constant.4. The method as recited in claim 1, wherein the amount X₁ is linearlydecreased and the amount of X₂ is linearly increased.
 5. The method asrecited in claim 1, wherein the metal oxide of the base slurry and thetransition slurry is selected from the group consisting of HfO₂, Y₂O₃,Yb₂O₃, Lu₂O₃, oxides of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, and combinations thereof.
 6. The method as recited in claim 5,further comprising applying a topcoat on the consolidated coating, thetopcoat being selected from the group consisting of HfO₂, Gd₂Zr₂O₇, andcombinations thereof.
 7. The method as recited in claim 1, wherein themetal oxide of at least one of the base slurry or the transition slurryis HfO₂.
 8. The method as recited in claim 1, wherein the metal oxide ofthe base slurry and the transition slurry is HfO₂.
 9. A gas turbineengine article comprising: a ceramic matrix composite substrate thatincludes SiC fibers disposed in a SiC matrix; a coating disposed on, andin contact with, the ceramic matrix composite, the coating comprising,by volume percent, 5% to 20% of barium-magnesium-aluminosilicate, and aremainder of a metal-silicon-oxygen rich phase and ametal-zirconium-oxygen rich phase dispersed through thebarium-magnesium-aluminosilicate.
 10. The article as recited in claim 9,wherein the metal-silicon-oxygen rich phase is HfSiO₄ and themetal-zirconium-oxygen rich phase is HfZrO₄ or HfO₂ and ZrO₂.
 11. Thearticle as recited in claim 9, wherein the coating has a porosity, byvolume, of 1% to 20%.